The soaring knife-edged towers of the National Gallery's new East Building have been visible for months. Passersby have peered into its huge windows to see the Calder mobile drifting overhead. I.M. Pei's building already has been acknowledged as a public monument. Now that its doors have formally been opened to the public by President Carter, it reveals itself as something else: an enormously successful place for seeing art.

Six important exhibitions went on view there yesterday, and it requires a masterpiece of architecture to be worthy of such art.

Each show is a unit, seen in isolation. High quality and great beauty are the only things they share.

"The Splendor of Dresden: Five Centuries of Art Collecting," an extraordinary loan show from the German Democratic Republic, surveys the treasures gathered by Saxony's electors, the insatiable collectors who ruled Dresden from the Middle Ages until 1918.

"American Art at Mid-Century: The Subjects of the Artist" offers one-man shows of seven American masters, each represented by a cycle of his work. The artists and their subjects are: Williem de Kooning's "Women"; Arshile Gorky's "The Plough and the Song"; Barnett Newmans's "Stations of the Cross"; Mark Rothko's "Brown and Gray" series (which were finished just before his death and have not been shown before); David Smith's "Voltri" sculptures; Robert Motherwell's "Elegies to the Spanish Republic," and Jackson Pollock, who is represented bythe "Classic" paintings that he poured and dripped 28 years ago.

"Piranesi: The Early Archictectural Fantasies" is a scholarly print exhibit honoring the Italian master on the 200th anniversary of his death.

"Master Drawings and Watercolors: Selections From the national Gallery's Collection and Promised Gifts" is an extraordinary survey of 800 years of draftmanship. Leonardo, Michelangelo, Durer, Rembrandt, Fragonard and Blake, Homer, Van Gogh and De kooning are among those represented all by first-rate works.

"Small French Paintiangs From the Alisa Mellon Bruce Collection" is a selection of 60 pictures, many of them tiny, all of them endearing, bequethed the gallery by Andrew Mellon's daugther, Alisa Mellon Bruce.

"Aspects of 20th Century Art" would be itself enough to fill a small musuem. It opens with a selection by Picasso and the cubists, and goes on to survey fauvism, futurism, expressionism, surrealism, constructivism and other major trends in 20th century abstract art. The "Aspects" exhibition closes with a generous reprise of the recent show of the cut-outs of Matisse. The gallery's five major cut-outs and an edition of the silkscreens that Matisse titled "Jazz" (borrowed from Paul Mellon) are here on display.

One enters the East Building into an "orientation space," a term that does not hint at the formal, airy grandeur of that huge, skylit hall. There are balconies and bridges here, pools, a grove of trees and overhead the sky. Paul Mellon, Pei and Carter Brown, who together planned the building, know that large musuems can overload the mind, and also test the feet. Pei's vast glass-roofed hall functions as a breathing space, an interval of light and clam between the exhibits. There are armchairs on its balconies, there will someday be a restaurant, its windows offer views of the Capitol, the Mall.

The skylit hall opens into galleries in many different shapes, finishes and sizes, Each suite of exhibition rooms seems a world unto itself. Some have walls of velvet, some have dark oak floors. The configurations of their walls can be easily adjusted to accommodate a postage-stamp, a mural or any work of art.

The Dresden exhibitions, by far the most spectular of the opening shows, will be on view all summer.

There are 700 objects in it - statuettes encrusted with diamonds and with rubies, Durer portraits, Weapons chased in gold, silver goblets, porcelains, suits of armor, posters - from the land of Bach and M* artin Luther.

This show is 15 times as large, and just as amazing, as the treasures of King Tut. It deals with five centuries, and it has been so installed (in the special exhibition spaces underneath Pei's hall) that to wonder through it is to move through time.

An alchemist would feel at home among the minerals and engraved tools, astrolabes and clocks, in the medieval Kunstkammer, or Cabinet of Curiosities, with which the show begins.

Soon the viewer finds himself among extraordinary weapons, lances, pistols, swords, whose military gallery seems part palace armory, part battle field tent. One moves from there into a re-creation of the Green Vault, the baroque mirroed treasury of the Electros of Saxony, whose display of pearls and diamonds, almost super-human craftmanship and incalculable wealth could drive a miser mad.

By the time the viewer passes through the gallery of porcelains, the room given to bronzes, and proceeds through the Old Master's room to the display of 20th-century German expressionist pictures with which the show concludes, he feels as if he's moved through five centuries of history, through libaries and wars, state visits, court festivities, not only through a show of extraordinary art.

There were only 125 paintings in the collections Andrew Mellon presented to the nations, with the National Gallery that houses them, in 1941. There are 100 times as many works in the new East Building, and their variety is great. The installation is superb, and the colors of the walls, the fabrics and the carpets, the labels and the lighting, all reflect the state of the art.

The 1,200 works of art, old as well as modern, are on loan both from millionaires and marxists. The exhibits, like the builking, are intentionally elitist. They exclude all but the best.

It is perhaps remarkable that those who made them possible - the architects, scholars, curators, collectors, the executive of IBM and the rulers of East Germany, and expecially Paul Mellon, the patron most responsible - could agree on this: There is no need to compromise. It is politically permissible toshu in the mediocre, at least in showing art.

Mellon, his late sister, and their family's foundation - aided by a tax system which encourages such gifts - gave us the East Building, at a cost of $94.4 million.

Mellon, a billonaire and a connoisseur of art, wine, jazz and horses insists on excellence not only in his private life but in his gifts to the public. Though he lent the East Building pictures (Picassos, a Matisse, a superb Van Gogh drawing), it was not built of his art. Others will determine what works it displays. The gallery reflects his aristocratic standards, not his private taste.

The National Gallery that his father built is a columned temple that celebrates the past. The East Building has, in contrast, been provided for the future. Of the paintings on display there, many date from our own century. It is, perhaps, worth noting that at its first evening preview, the after-dinner music was Benny Goodman's jazz. Almost all of its exbits stress the continuity between old and modern art.

The Ailsa Mellon Bruce exhibit of French paintings, is, and rightly so, as intimate and modest as the Dresden show is grand. Although Mrs. Bruce bought the National Gallery a number of its most important pictures (among them America's only Leonardo), she was not herself an avid art collector. Still, she graced her New York townhouse with dozens of sweet paintings, most of them French. She preferred them small. Her pictures do not conquer walls, but these Vuillards and Boudins, Renoirs and Monets,Seurats and Bonnards are pictures one would love to live with. The East Building may be monumental, but it effortlessly accepts these paintings of small size.

Pei's galleries do just as well with the drip paintings of Pollock. Anyone who wonders why the gallery, and the Australians, have been willing in recent years to spend as much as $2 million for paintings by that master, will find the answer here. In an adjacent gallery, whose plan is a square spiral. Newman's "Stations of the Cross" march with painful power.

Of the seven galleries in the "Subjects" exhibition, none is more impressive than that given to the Smiths. Unprecedented works of endless energy and wit, all of them were made for the Spoleto Festival in the little town of Voltri in 1962. Here they are displayed in a kind of skylit amphitheater they recalls the ancient ruins among which they were first seen. No section of the building is more successful than that gallery. It is a place of beauty, and of theater, too.

Drawings, it is said, are an acquired taste, but no visitor should miss the Master Drawing show that is just to the left of the front door. Here we see that Ingres made as flawless pictures with pencil as with paint. Winslow Homer's watercolors, with their brilliant painted sunlight, are the equal of his oils. And Paul Mellon's Van Gogh landscape, though done in pen and ink, seems in memory as colorful as a picture made with the brightest colored paints. The Dresden show will travel, the Smiths will be returned to those who own them, but these 120 drawings always will be here.

The "Aspects" exhibition is similarly mouthwatering. All of the objects in it are from the gallery's collection, or lent by collectors "who have shown a special interest" in the new museum.

If, as may be hoped, some of these major works eventually enter the gallery's collection, the museum will no longer seem an institution that neglects the present in favor of the past.

The Mellons have given us an astonishingly good building in hope that other collectors would someday fit it up with art. It is because that happaned to the gallery next door - founded by their father - that the East Building was constructed.

The National Gallery of Art opened in 1941 with its few fine pictures installed far apart. Though many of its works are borrowed, the East Building is, though new, already one of America's most beautifully designed and thoughtfully installed galleries of art.